The present invention relates to a headbox for a paper machine, and more specifically to a headbox including a stock flow aligning device having means defining a plurality of flow channels arranged in rows extending substantially in the cross machine direction, each of said channels enlarging stepwise in the direction of stock flow therethrough.
The use of hole plates in headboxes for paper machines and similar machinery has long been known. To align the flow and eliminate cross flow tendencies, such hole plates have been made thicker and thicker (see e.g. U.S. Pat. No. 3,725,197) and have developed into blocks having channels drilled therethrough, suitably widened stepwise in the direction of stock flow. Such blocks have sufficient rigidity to take up the load from the stock pressure equally across the headbox dimension in the cross machine direction, so that changes in load do not cause unequal changes in the machine geometry.
Due to the size and weight of the block and the costs involved, attempts have been made to replace them by banks of tubes of considerable length in relation to their diameter, the opposite ends of the tubes being firmly clamped in tube plates disposed in fixed, spaced apart relation to each other. If the paper machine is started with the stock at a considerably different temperature than the tube bank, as is often the case, differential expansion is likely to produce resultant forces great enough to deform the headbox and cause small, but vital, changes in the geometry of the headbox slice opening. Before steady conditions are attained, which can take 8-12 hours, the paper machine will be difficult to control, the quantities of broke large, and the frequency of web breaks increased, resulting in production losses, rejects, and unsatisfactory operation. In addition, the resultant forces can be of the same order of magnitude as the forces retaining the tube ends in the plates, so that tube end clamping can fail and result in leakage.